Downstream on homebrew we ran into the issue that on OS X Mavericks 10.9 you would really want to build with libc++ by default. In particular when linking against system libs (e.g. boost) this is a requirement, since they are likely built with libc++. Could we change the default arguments for OS X to the following:

a) On 10.7 and above, if --libc++ is passed, default osx-version-min to 10.7. (currently the default is 10.6, which conflicts with libc++ such that supplying the libc++ flag alone has no effect)
b) On 10.9 and above, default to using libc++ (and thus also osx-verison-min 10.7)